File:The causes and course of organic evolution; (1918) (14750980566).jpg

Identifier: causescourseofor00macf (find matches)
Title: The causes and course of organic evolution;
Year: 1918 (1910s)
Authors: Macfarlane, John Muirhead, 1855-1943
Subjects: Evolution
Publisher: New York, The Macmillan Company
Contributing Library: MBLWHOI Library
Digitizing Sponsor: MBLWHOI Library

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onsive tothe earths mass, while it is also heliotropic to light, and thusproenvirons a course that is in line with the light rays andgravic stimulus. But, if the well-known experiment of illum-inating a crop of it through a minute orifice on one side of anotherwise darkened chamber be arranged, then the heliotropiclesponse is so great that the apogeotropic one is canceled ormodified to an angular degree that accurately brings the axis Law of Proenvironment 213 of the sporangiophore in line with the incident rays. So insuch an illustration as that of Fig. 5 each individual plotsand pursues a proenvironal path that is different in angularrelation to every other one. But all execute such movementthat the most beneficial result is secured to the species, inresulting growth and dissemination of the spores. The sur-prisingly accurate manner in which the sporangia hit a circum-scribed light-orifice is a striking proof of lines of energy-dis-tribution and of cell tensions in relation thereto.
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Fig. 5.—Growth of the Fungus Piloboliis on horse manure, b, placed underbelljar, a, the exterior of which was lined with black paper except for orifice atc. From resultant action of apogeotropic and specially of heliotropic stimulithe black sporangia are discharged accurately against the orifice. But the feeding or absorptive threads of this fungus, whichare a continuation of the same elongate plant cell that producedthe spore masses, exhibit proenvironal responses of a totallyopposite kind, though equally beneficial for the species. Thusthese grow either directly or obliquely downward into themanure through geotropic action; they at the same time growaway from the light or are apoheliotropic; they also are influ-enced hydrotropically by the moisture of the manure and sogrow toward it to absorb food material. All three of these 214 Causes and Course of Organic Evolution separate responses are usually compounded in a resultant lineof growth, that is nearly or quite the same as for an

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